Light-sensitive coating and the method for its production



United States Patent LIGHT-SENSITIVE COATING AND THE METHOD FOR ITS PRODUCTION Thomas R. Caton, Lynbrook, N .Y.

No Drawing. Application April 8, 1958 Serial No. 727,032

20 Claims. (Cl. 96-93) This invention relates to a coating composition, a method for its production and a light sensitive film produced therefrom, and to a dry mixture adapted for solution in dilute aqueous ammonium hydroxide to produce the coating composition. More particularly, it relates to a coating composition and to a light-sensitive film which is adapted for use'in the production of photolithographic off-set printing plates.

Photo-lithographic off-set printing plates are usually of aluminum or zinc and have specially prepared surfaces, some areas of which are capable of accepting an ink having an oily vehicle and other areas which, when wet with water, will not accept ink. The area of the plates which accept ink form the printed image. Such plates must be constantly wet with water or an aqueous solution during the printing operation.

The initial step in the preparation of such a plate is a graining operation by which the surface of a metal plate is given a matte texture consisting of microscopic hills and valleys. This grained surface is then coated with a light-sensitive film and exposed through a negative of the image to be printed to an actinic light to produce a chemical reaction in the exposed portions of the film which renders them substantially insoluble in water. The unexposed portions of the film are then removed from the plate, leaving the light-hardened portions on the image areas of the plate to form its ink-receptive printing areas of the plate.

Casein is produced commercially by the treatment of milk with a mineral acid to cause the separation of the casein as curds from the whey of the milk. The Whey is drained from the curds, which are then washed with water and dried. The dried casein is soluble in an aqueous solution of ammonia and is widely used in the form of an ammoniacal solution containing dissolved ammonium dichromate as a light-sensitive coating composition for the production of lithographic printing plates.

Photo-lithographic printing plates produced by the use of such light-sensitive coating are of limited durability, necessitating the preparation of duplicating plates for extended press runs. Furthermore, they do not reproduce half-tone images with the clarity achieved, for example, with plates made by photo-engraving processes. This relatively poor clarity is due to the matte texture of the grained surface underlying the image reproducing areas of the plate.

The preliminary graining of the metal has heretofore been absolutely essential in the preparation of photolithographic printing plates to obtain the selective wettability essential to this type of printing, by causing the non-image forming areas of the plate to retain more water than a smooth metal surface will retain. The quantity of water which is retained by this type of plate renders it unsuitable for the printing of surfaces which have a limited capacity for absorbing water, such as, for example, a cellulose acetate foil. Furthermore,

2,921,852 Patented Jan. 19, 1960 the graining operation materially increases the cost of this type of printing. 7 i

The graining operation is a delicate one which, in present commercial practice is usually carried out by firms specializing in such work, rather than in the laboratories in which the plates are prepared for printing. It is an operation which mustbe repeated each time the metal plate is reused in the preparation of a printing plate. These factors require that the processing laboratories maintain a relatively large inventory of the plates to permit economy in the packing of. used plates for regraining. and require repeated trans-shipment of the plates. Further, each graining operation to which a plate is subjected removes metal from its surface, with the result that it can be regrained, only a limited number of times before it becomes useless due to rupture'at its gripper edges. Each of the factors increases the cost of the printing operation.

Now, in accordance with this invention, I provide a coating composition which dries to a light-sensitive film from which image forming areas can be produced which are substantially more durable than those produced heretofore. My coating composition is suitable for use in the production of printing plates by the use of grained metal plates and is, in addition, suitable for the production of printing plates by the use of smooth, ungrained plates of a variety of different metals, includdin'g-alw minum, zinc, tin plated steel, and stainless steel, Photo'- lithographic off-set printing plates prepared by the use of ungrained metal plates by the use of my coating composition'avoid the expense of the graining operation and are capable of reproducing half-tone images with a clarity which has been ullQbtainable by the use of printing plates having grained surfaces. p

This invention is the result of research which has disclosed two facts. It has demonstrated that the treatment of an ammoniacal dispersion of casein, either in the presence or absence of dissolved ammonium dichromate, with an amount of a mineral acid insufiicient to fully neutralize the ammonium hydroxide present in the solution, causes the separation of an organic material from the solution which appears to bea selective fraction of the casein itself. A '3 This research has further demonstrated that the portion of the casein which remains in the ammoniacal solution, after the removalof the fraction which is separated by the acid treatment described above, is suitable for the production of a lithographic printing plate which is materially more durable and capable of producing a materially greater number of copies then one produced by the use of a casein which has not been subjected to the fractionation treatment.

The foregoing facts have led me to the conclusion that this acidic fractionation of the casein removes a portion thereof which tends to weaken the image on a lithographic printing plate in which it forms the major filmforming constituent. Conversely stated, the removal of this particular fraction, materially increases the durability of the image and, hence, the number of copies which can be produced by the use of the printing plate.

The coating composition, in accordance with this invention, comprises an ammoniacal solution of ammonium dichromate having dispersed therein milk casein from which a minor fraction has been removed by the addition of a concentrated mineral acid, to an ammoni acal solution of the casein in an amount less than the stoichiometric quantity necessary to completely neutralize the ammonium hydroxide present in the solution. In general, the fraction of the milk casein which has been removed by .the treatment with a mineral acid is by weight, of the casein originally present in the solu in the ammoniacal coating composition.

tion, and is preferably about by Conversely stated, the casein fraction contained in this coating composition is about 98%, by weight, to about 8.5%,by weight, and, preferably about 95 by weight, ofacommercial' milk casein. This coating composition preferably contains about one part of ammonium dichromate to about 8 parts of the acid fractionated casein. i Infaddition to ammonium dichromate, .theacid-fractionated casein, ammonium hydroxide and water, this coating composition will normally contain the ammonium salt of the mineral acid which is utilized in the fractionation of the casein. Mineral acids which may be used forthe fractionation of the casein are, for example, chromic acid and nitric acid. In general, 'I prefer to use chromic acid for thefractionation. of the casein, since the residuaI salt leftinthe coatingcomposition is an ammonium chromate orammonium dichromate, i.'e. the same salt which is present as the light sensitizin'g agent I By the method in accordance with this invention, for the production of my'improved' coating composition, I prepare an aqueous ammoniacal dispersion of milk casein and add thereto an amount of a concentratedmineral acid at least suflicient to cause the precipitation ofa minor proportion of the dispersed milk casein, but less'than the stoichiometric quantity required to neutralize all the ammonium'hydroxide present in the solution, and thereby render it non-alkaline inreaction. I ordinarily use an amount of the mineral acid which will cause the precipitation of an amount of solids within the range ofabout 2% to about 15% by weight, and preferably about 5%, by weight, of the milk casein dispersed in the solution. I then remove the precipitated solids from the solution. After the precipitated solids are removed from the solution, I prefer to return the pH of the solution to substantially its original value, prior to the addition of the mineral acid, for example, pH 9.0, by the addition of concentrated ammonium hydroxide in amount to substantially replace that which has reacted with the mineral acid. The other ingredients which I include in the completed composition may be added to the ammoniacal dis persion of milk casein either before or after the mineral ,acid. treatment, although in practice I prefer to add them prior to the acid treatment. Thus, I prefer to prepare a composition Whichis, in efiect, a prior art light-sensitive coating composition comprising essentially an aqueous ammoniacal solution of ammonium dichromate in which milk casein is dispersed and then subject it to treatment which a mineral acid and remove a precipitated fraction of the. casein as described hereinbefore.

The ingredients which I add to the ammoniacal casein solution, either before or after the mineral acid treatment, essentially includerammonium dichromate; I may, in addition, include a minor amount of dispersed gelatin, forexample, about 0.02%, by weight, of the solution and a small amount of a water-soluble organic dye, for example, an amount within the, range of about 0.1% to about 0.3%, by weight, of the solution.

'Ihelight-sensitivefilm in accordance with this invention comprises a uniform molecular dispersion of ammonium dichromate in a film comprised essentially of milk casein, at least the milk casein component ofwhich has been -t reated while dispersed in -a dilute aqueous ammonium hydroxide solution to precipitate a minor fraction of the casein which is separated from the solu" tion. My preferred film comprises about 12 parts, by weight, of ammonium dichromate for each 80 parts, by weight,-of the fractionated casein. The him may also carry a molecular dispersion of a water-soluble dye, for example, an amounuwithin the range of about one to about 4 parts,'by weight, preferably about. 2.5 parts, by weight, -for'each 80 partsof the fraction of milk casein present therein. .The 'filn'i niay 'also contain a minor quantity of gelatin, forexample, about 0.2 part, by weight,-

weight, thereof. for each"80 parts, by weight, of

the said casein. It will also contain an ammonium salt formed as a byproduct of the mineral acid treatment of the milk casein present.

The coating composition in accordance with this invention and all coating compositions heretofore used for the formation of light-sensitive films adapted for the production of photo-lithographic off-set printing plates are aqueous solutions having a relatively high water content, usually eighty percent or more of their total Weight. They are corrosive in nature and are packaged in glass containers. These factsm'ake the distribution of such coating compositions extremely expensive, when considered in terms of their actual content of film-forming solids which form'the light-sensitive films.

In accordance with this invention I provide a dry mixture which is readily dispersible in a dilute aqueous solution of ammonium hydroxide to produce a coating composition which dries to a light-sensitive film adapted for the production of a photo-lithographic printing plate. This dry mixture comprises essentially ammonium dichromate and fractionated'milk, casein, in a uniform molecular dispersion in each particle of the solid. In addition to the ingredients mentioned, this. solid mixture preferably contains a minor quantity. of gelatin anda watersoluble, organic dye molecularly dispersed throughout each particle of the solid. I preferto produce it in the form of a powder, from the standpoint of'convenience in dispersing it indilute aqueous ammonium to. produce a coating composition This'dry powder, when dissolved in dilute aqueous ammonium hydroxide, produces the improved coatingcomposition in accordance with this invention.

I produce this molecularly dispersed dry mixture by the evaporation of the ammonia and the water from the coating composition of this invention, leavingas a residue its content of fractionated milk casein, ammonium dichromate or ammonium chromate. as the case may be, gelatine. and eosindye as a molecularly dispersed solid. In evaporating the ammonia and water from this solution I prefer to utilize an elevated'temperature, but not in excess of 130 C. and preferably not in excess of 110 C. and to expose the solution to the highertemperatures for a minimum length of time; since a prolonged heating of the solution at the higher temperatures renders the dry powder partially insoluble in dilute aqueous ammonium hydroxide and therefore, unsuitable for my purpose. I have found that a double drum dryer heated by low pressure steam is satisfactory for carrying out this evaporation step; 7

As will'be; appreciated from the foregoing, to produce the dry, molecularly dispersed mixture of thisinvention,

, I first produce-my improvedrlight-sensitiv'e coating composition, comprising essentially an aqueous, ammoniacal solution of ammonium dichromate or ammonium chromate and of. fractionated casein, and 'then remove the water and ammonia therefrom under controlled condiv tions to produce a solid, preferably in finely-divided form,

which is freely soluble in an aqueous solution' of 'ammonia; The solution of this solid material in aqueous ammonia reconstitutes the original coating composition of this invention. f j

In preparing the coating compositioniin accordance with this invention, I find that it is desirable: to prepare base solutions of its various ingredients and then prepare an intermediate mixture by mixing these base solutions. This procedure is convenient in practical operations, produces complete dispersion of each essential ingredient under the .most favorable conditions. and avoids adverse effects due to localized high concentration of individual ingredients in the mixing operations. Thus, I may utilize three base solutions in the preparation of my intermediate mixture. These base solutions are an ammoniacal solu tion of milk casein, an aqueous'gelatin solution, and an ammoniacal solution of ammonium dichromate. I have found that it is advantageous to have the light-sensitive film formed from my finished coating composition colored to render it more readily visible. It may be colored with any water soluble, organic dye which is stable in aqueous ammoniacal solutions. I have found that it is convenient to add the dye to my intermediate mixture as a fourth base solution, although it may, if desired, be added as a final step in the completion of my coating composition.

A casein base solution which I have found convenient for use in the preparation of my coating compositions is shown by Example I.

To prepare this base solution, I stir the milk casein into approximately three-quarters of the water content of the composition and mix for a period of about 40 minutes while raising its temperature to about 135 F. I then discontinue the heating of the mixture, add the ammonium hydroxide and continue the mixing for an additional period of about 45 minutes. I then add the remainder of the water to the composition and briefly continue the mixing to secure a uniform consistency. It is desirable to use a high quality milk casein in preparing this base solution. I have found that one sold under the trade name Protovac is suitable for my purpose.

A gelatin base solution for admixture with the foregoing casein base solution may, for example, have the composition shown by Example 11.

i Example 11 Gelatin base solution: Percent by weight Gelatin 0.2 :Water 99.8

Example III sensitizing base solution: Percent by weight Ammonium dichromate 7.86

Ammonium hydroxide (27% NH 17.21

Water 74.93

This composition is prepared by merely' mixing the components.

I have found that eosin is a suitable water soluble organic dye for use in my coating. A suitable base solution of eosin is illustrated by Example IV.

Example IV Dye base solution: Percent by weight Eosin 4.3 Water 95.7

To produce my intermediate mixture, I mix the base solutions illustrated by Examples I, II, III and IV in the volumetric proportions shown by Example V.

Example V Parts by volume Casein base solution (Example I) Gelatin base solution (Example II) ll Sensitizingsolution (Example HI) 2 1 Dye base solution (Example IV) 8 The admixture of these different base solutions as illustrated by Example V produces an intermediate solution having the approximate percentage composition shown by Example VI.

The solution illustrated by Example VI, although further treated as exemplified hereinafter to produce the improved coating composition in accordance with this invention, is itself a valuable coating composition and well suited for commercial use.

To produce the improved coatingcomposition in accordance with this invention from the intermediate solution exemplified by Example VI, I add to this solution a concentrated mineral acid in an amount less than the stoichiometric quantity necessary to fully neutralize the ammonia present and then remove solid material which separates from the solution. Thus, for example, I may add an amount of concentrated nitric acid within the range of about 25 ml. to about 35 ml. and preferably about 30 ml. per gallon of solution. This addition of nitric acid causes the separation of a slimy, gelatinous precipitate from the solution which I believe to be a fraction of the milk casein originally dispersed therein. This precipitate is then removed from the solution by filtra-' tion. The pH of the intermediate solution exemplified by Example VI is about 9.0. The addition of 30 ml, of concentrated nitric acid (42 Baum) reduces this pH slightly to about 8.8. After I remove the solid material which precipitates upon the addition of the nitric acid, I return the pH of the solution to its original value of about 9.0 by the addition of concentrated aqueous ammonium hydroxide.

The composition of the completed coating composition in accordance with this invention is illustrated by Example VII. This particular composition was produced by the addition of 30 ml. of concentrated nitric acid (42 Baum) per gallon of an intermediate solution having the composition illustrated by Example VI followed by the separation of the precipitated solids and the readjustment of its pH to a value of 9.0 by the addition of concentrated ammonia.

Example VII Coating composition:

A comparison of Example VII with Example VI will show that the removal of the precipitate from the inter mediate solution of Example VI has reduced the casein Percent by weight content of the solution from 8.28% by weight, to 7.91%, by weight. Thus, the precipitation step removed 4.47% of the casein originally dispersedin the solution, in the case of this particularfexemplification of my method.

Before using the "coating composition' illustrated by Example VII for'theproduction of a' photo-lithographic off-set printing plate,I 'diluteit with'water. it to produce a light-sensitive film on an ungrained metal plate, I dilute it by the addition of an equal volume of water, thereby "reducing its film-forming solids content by 50%. When using it to produce alight-sensitive film on a grainedmetal plate I reduce its film-forming solids content by 25% by the addition ofwater. These dilutions with water reduce the thicknessof the light-sensitive film obtained by drying the composition, but does not alter w it qn The composi on of alight-sensitive film in accordance with this inyention, produced bydrying the coating composition exemplified by Example YII, regardless of the extent of its dilution with water, is shown'by Example Example VIII Light-sensitive film: Percent by Weight Nitric acid treated casein 77.09 Ammonium dichro'mate 11.40 Ammonium nitrate 8.98 Gelatin 0.16 Eosin dye 2.37

' steel, I counter-etch the plate before depositing'thelightsensitive film thereon. In the case of aluminum plates, I use a counter-etch of dilute acetic acid, for example, one containing 4 ounces by volume of glacial acetic acid per gallon of Water. tioned, I usea dilute solution of hydrochloric acid as a counter-etch. A solutio'n containing 1 ounce, by volume, of concentrated hydrochloric acid per gallon of water is suitable.

In the case of smooth metal plates, I prefer to rub the counter-etched plate to a smooth satiny finish by the use of a composition containing mild abrasive, such as, for

example, pumice powder, and follow this treatment by a second counter-etch. The satiny finish which I prefer to give the smooth metal plates is not to be confused with the grained. surface which has heretofore been considered absolutely essential tothe production of a satisfactory photo-lithographicoif-set printing plate, since the extent of the abrasioninvolved in the .two cases are of totally different orders of magnitude and of character. V i

I form the light-sensitive film, in'accordance with this invention, o'n the metal plate by the usual procedure of pouring it on a plate being whirled in a horizontal plane and continuing the whirling until the film isv dry..

The coating composition exemplified by Example VII may be dried to produce the molecularly dispersed dry mixture in accordance with this invention. The. drying operation,-may for example, be carried out as described hereinbefore. The composition resulting from the drying of the coating composition of Example VII is; identical with that of the light-sensitive -film shown by Example To produce a co'ating composition for use in the production of photo-lithographic printing plates from this dry compositiomI merely dissolve the composition in dilute aqueousammoniai 'I;may, for example, addlO When using In the case of the other metals menparts, by weight, of the solid to about 86 parts, by weight, of an aqueous solution of ammonium hydroxide containing 1%, by weight, of ammonia, and thereby produce a coating composition approximating that exemplified by Example VH. It maybej dissolved in 172 parts of such a dilute solution {of aqueous ammonium hydroxide to producejacoating composition ready for use on an ungrained metal'plate or in 108 parts of such a dilute solution of ammonium hydroxide to produce a coating composition suitable for use on a grainedmet'al-plate.

In the foregoing, the various embodiments; 'of this invention have been specifically exemplified by compositions in which a milk caseinwhich has been fractionated by the use. of nitric acid is used as the major film-forming constituent- Althoughnitric acid is one ofthe preferred mineral acids'for use in the fractionation of casein in accordance with this invention, it willbe understood from the foregoing that the other strong mineral acids are effective in the fractionation of casein, and that the use of casein fractionated bytheir use results in compositions which are definitely superior to the corresponding prior art compositions. In utilizing the strong mineral'acids other than nitric acid, I substitute for the nitric acid used in the examples given hereinbefore, the stoichiometric equivalent of the particular mineral acid which I desire to use.

My research has'demonst'rated that the use of nitric acid for the fractionation of milk casein produces outstanding results, and that the compositions prepared by the use of the casein so fractionated are greatly superior to the prior art compositions. This research'has also shown that chromic acid is even better than nitric acid for this purpose. It is at least as effective as nitric acid in the fractionation of milk casein to remove the fraction which weakens the image on a lithographic printing plate, and has the advantage of'leaving as a residual salt ammonium chromate or ammonium dichromate, as the case may be, i.e. exactly the same salt which is added to the compositions to render them light-sensitive. As brought out hereinbefore, when nitric acid is used in my process, the final compositions contain residual ammonium nitrate resulting from the neutralization of the nitric acid by the ammonium hydroxide in the composition.

When using chromic acid as the fractionating acid, I may prepare a completed light-sensitive coating composition of the prior art type, add chromic acid thereto, remove -a separated fraction of organic material therefrom and then readjust the pH of the composition by the addition of concentrated aqueous ammonia. Alter-, natively, I may prepare an ammoniacal solution of milk casein, fractionate it by the addition of chromic acid, readjust thepH of the solution by the addition of concentrated aqueous ammonia and then add ammonium dichromate and any other auxiliary vingredients;Which I.

desire in the final composition. 'Thus, Iutilize chromic acid in exactly the same manner in which I utilize nitric acid or another of the strong mineral acids. Chromic acid is handled commercially in the'form' of chromic anhydride and I fiind that I can conveniently add chromic anhydride directly to the ammoniacal solution of milk casein in my'method. i

In utilizing chromic acid as the strong mineral acid in my method I may, for example, add three pounds of chromic anhydride to sixty-five gallonsofthe coating composition illustrated by Example VI. In carrying out this addition I find that it is desirable to agitate the coating composition solution in a manner. which causes it to form a vortex, and pour the chromic anhydride into the vortex. After such addition of the chromic anhydride, soft lumps of organic material separate from the solution, which can be readily removed therefrom merely by straining thesolution by passing it through a cloth.

Theactual separation of a fraction of the casein in my method is clearly responsible for the improved properties of my compositions, rather than'the presence of the 9, residual ammonium salt of the strong mineral acid present in the composition. My research has shown that a light-sensitive coating composition containing casein which has been fractionated with chromic acid, will produce a lithographic printing plate which will produce many more copies than a composition which is identical except in that it contains an unfractionated casein as its major film-forming constituent. Again, this research has shown that in the use of" nitric acid as a fractionating acid, the. fractionation of the casein is responsible for the increased durability of the lithographic printing plates made from the light-sensitive coating composition containing the fractionated casein, since the use of a light-sensitive coating composition utilizing ordinary milk casein to which an amount of ammonium nitrate equivalent to that produced by the neutralization of the nitric acid in my process and which is identical in composition to that containing the fractionated casein. except as to its casein content, does not produce a lithographic printing plate having the greatly increased durability shown by the plates made by the use ofthe light-sensitive coating composition-made with the fractionated casein.

In the foregoing, many details have been given as to the method and the products in accordance with this invention and both the method and the-products have been specifically exemplified. It will be fully understood that the details and specific examples have been given for the purpose of fully disclosing my invention and that various modifications and changes can be made in these details without departing from the spirit of my invention or the scope of the appended claims.

This application is a continuation-in-part of my copending application Serial No. 470,118, filed November 19, 1954.

I claim:

l. A light-senstive film adapted for use in the production of photolithographic off-set printing plates which comprises a molecular dispersion of ammonium dichromate in a film comprising essentially a milk casein which was originally produced by precipitation from milk and treated with nitric acid while uniformlydispersed in an ammoniacal aqueous solution and had a minor part precipitated and removed from the said solution.

2. A light-sensitive film adapted for use in the production of photolithographic off-set printing plates which comprises a molecular dispersion of amornnium dichromate and of a Water soluble organic dye in a film comprising essentially a minor proportion of gelatin and a milk casein which was originally produced by precipitation from milk and has been treated with nitric acid while uniformly dispersed in an ammoniacal solution and had a minor part precipitated and removed from the said solution.

3. A light-sensitive film adapted for use in the production of photolithographic off-set printing plates which comprises a molecular dispersion of about 12 parts, by weight, of-ammonium dichromate in a film comprising essentially about 80 parts, by weight, of a milk casein which was originally produced by precipitation from milk treated with nitric acid while uniformly dispersed in an ammoniacal aqueous solution and had a minor part precipitated and removed from the said solution.

4. A light-sensitive film adapted for use in the production of photolithographic ofi-set printing plates which comprises a molecular dispersion of about 12 parts, by weight, of ammonium dichromate in a film comprising essentially about 80 parts, by weight, of a milk casein which was originally produced by precipitation from milk treated with a strong mineral acid selected from the group consisting of nitric acid and chromic acid while uniformly dispersed in an ammoniacal aqueous solution and had a minor part precipitated and removed from the said solution.

5. A light-sensitive film adapted for use in the production of photolithographic printing plates which comprises a molecular dispersion of about 12 parts, by weight, of ammonium dichromate ina film comprising-essentially about parts, by weight, of a milk casein which was originally produced by precipitation from milk treated with chromic acid while uniformly dispersed in an ammoniacal aqueous solution and had aminor part precipitated and removed from the said solution.

6. A light-sensitive film adapted for use in the production of photolithographic off-set printing plates whichcomprises a molecular dispersion of about 12 parts, by weight, of ammonium dichromate and a water soluble organic dye' within the range of about 1 to about 4 parts, by weight, in a film comprising about 0.2 part, by weight, of gelatin and about 80 parts, by weight, of a milk casein which was originally produced by precipitation from milk and treated with nitric acid while uniformly dispersed in an ammoniacal aqueous solution and had a minor part precipitated and removed from the said solution.

7. A powdered solid adapted for solution in a dilute aqueous solution of ammonium hydroxide to produce a coating composition for use in the production of photolithographic printing plates, each particle of which comprises a molecular admixture of ammonium dichromate and a milk casein whichwas originally produced by precipitation from milk and treated with a strong mineral acid selected from the group consisting of nitric acid and chromic acid while uniformly dispersed in an ammoniacal aqueous solution and had 'a minor part precipitated and removed from the said solution.

8. A powdered solid adapted for solution in a dilute aqueous solution of ammonium hydroxide to produce a coating compsoition'for use in the production of photo lithographic off-set printing plates each particle of which comprises a molecular admixture'of ammonium dichromate, and a milk casein which was originally produced by precipitation from milk and treated with nitric acid while uniformly dispersed in an ammoniacal aqueous solution and had a minor part precipitated and removed from the said solution.

9. A powdered solid adapted for solution in a dilute aqueous solution of ammonium hydroxide to produce a coating composition for use in the production of photolithographic printing plates, each particle of which comprises a molecular admixture of ammonium dichromate and a milk casein which was originally produced by precipitation from milk and treated with chromic acid while uniformly dispersed in an ammoniacal aqueous solution and had a minor part precipitated and removed from the said solution.

10. A powdered solid adapted for solution in a dilute aqueous solution of ammonium hydroxide to produce a coating composition for use in the production of photolithographic off-set printing plates, each particle-of which comprises a molecular admixture of ammonium dichromate, a water soluble organic dye, a minor proportion of gelatin and 'a milk casein which has been treated with nitric acid while uniformly dispersed in an ammoniacal solution and had a minor part precipitated and removed from the said solution, the said molecular admixture being the product resulting from the removal of water and ammonia from an aqueous ammoniacal solution of the said ingredients of the said molecular admixture.

11. A powdered solid adapted for solution in a dilute aqueous solution of ammonium hydroxide to produce a coating composition for use in the production of photolithographic printing plates, each particle of which comprises a molecular admixture of ammonium dichromate, a water soluble organic dye, a minor proportion of gelatin and a milk casein which has been treated with a coating composition'for use inthe production of photolithographic printing plates, each particle of which comprises a molecular admixture of' ammonium dichromate,

a water soluble organic dye, a minor proportion of gela: tin and a milk casein which has been treated with chromic,

acid while uniformly dispersed in an ammoniacal aqua ous solution and had a minorpart precipitated and removed from the said solution, the said molecular admixture being the product resulting from the removal of water and ammonia from-an aqueous ammoniacal: SO11}? tion oi the. said ingredients of the said molecular admixture.

13. A powdered solid adapted for solution in a dilute aqueous solution of ammonium hydroxide to produce a coating composition for use in the production of photolithographic olf-set printing plates, each particle of which comprises a molecular admixture of about 12 parts, by weight, of ammonium dichromate and about 80, parts,

i by weight, of-a'milk casein :which was originally produced by precipitation from milk-treated with nitric acid while uniformly: dispersed in an ammoniacal aqueous solution and had a minor part precipitated and removed from the said solution, the said molecular admixture being the product resulting from the removal of water,

and ammonia from an aqueous ammoniacal solution of 'the said ingredients of the said molecularadmixture.

14. A powdered solid adapted for solution in a dilute aqueous solution of ammonium hydroxide to produce a coating composition for use in the production of photolithographic ofi-set printing plates, each particle of which comprises a molecular admixture of about 12 parts, by weight, of ammonium dichromate and a water soluble organic dye within the range of about 1 to'about 4 parts, by weight, in a film comprising about 0.2 part, by weight, of gelatin and about 80 parts, by weight, of a milk casein which'was originally produced, by precipitation from milk and treated with nitric acid while .uniformly dispersed in an ammoniacal aqueous solution and had a minor part precipitated and removed from the said solution. a

15. A method for the production of a coating composition adapted for use in the production of photolithographic off-set printing plates which comprises forming an ammoniacal aqueous solution comprising essentially milk casein and ammonium dichromate adding concentrated nitric acid thereto in an amount sutficient to cause theprecipitation of a minor proportion of the dispersed milk casein but insufficient to render the said solution as a whole non-alkaline and separating the precipitated casein from the said solution; V v V 16. A method for the production of a coating composition adaptedior use in the production of photolithographic printing plates which comprises forming an ammoniacal aqueous solution comprising essentially milk casein and ammonium dichromate adding a concentrated strong mineral acidselected from the group consisting said ingredients of the,

12 of nitric acid and chromic acid thereto in an amount sufficient to cause the precipitationof atm inor proportion of the dispersed milk casein but insuflicient torender the said solution as a whole non-alkaline and separating the precipitated casein from the said solution.

17. A method for they production of a zcoating composition adapted for use in the production of photolithographic printing plates which comprises forming an an ammoniacal aqueoussolution comprising essentially milk casein and ammonium dichromate adding concentrated chromic anhyd'ride thereto in an amount suflicient to cause the precipitation of a minor proportion of, the dispersed milk casein but insufficient to render the said solution as a whole non-alkaline and separating the precipitated casein from the said solution.

18. A method for the production of a coating composition adapted for use'in'the production of photolithographic off-set printing plates which comprisesforming'an ammoniacal aqueous solution comprising essen tially milk casein and ammonium dichromate adding concentrated nitric acid thereto in an amount 'sufficient to cause the precipitation of aminor proportion of the dispersed milk casein but insuflicient to render the said solution as a whole non-alkaline separating the precipitated casein fiom the said solution and adding concentrated ammonium hydroxide to the said solution'in an amount suflicient to substantially replace that whichhas reacted with the said nitric acid.

19. A method for the productionof a coating composition adapted for use in the production. of photolithographic printing plates which comprises forming an anmioniacal aqueous solution comprising essentially milk casein and ammonium dichromate adding concentrated strong mineral acid selected from the group consisting of nitric acid and chromic acid thereto in an amount sufficient to cause the precipitation of a minor proportion of the dispersed milk casein but insuflicient to render the said solution as a whole non-alkaline, separating the precipitated casein from the said solution and adding concentrated ammonium hydroxide to the'said solution in 'an amount suflicient to substantially replace that which has reacted with the said strong mineral acid.

20. A method for the production of a coating composition adapted for use inthe production of photolithographic ofiE-s'et printing plates which comprises forming an ammoniacal aqueous solution comprising essentially milk casein, and ammonium dichromate adding concentratednitric acid thereto in amount sufficient to causethe precipitation of about 2%, by weight, to about 15%, by weight, of themilk casein dispersed in the said solution, while leaving the said solution as a whole still alkaline separating the precipitated casein from the said solution and adding concentrated ammonium hydroxide to the solution in amountsuflicient to substantially replace that which has reacted with the said nitric acid.

References Cited in the file of this patent UNITED STATES PATENTS 

7. A POWDERED SOLID ADAPTED FOR SOLUTION IN A DILUTE AQUEOUS SOLUTION OF AMMONIUM HYDROXIDE TO PRODUCE A COATING COMPOSITION FOR USE IN THE PRODUCTION OF PHOTOLITHOGRAPHIC PRINTING PLATES, EACH PARTICLE OF WHICH COMPRISES A MOLECULAR AS MIXTURE OF AMMONIUM DICHROMATE AND A MILK CASEIN WHICH WAS ORIGNALLY PRODUCED BY PRECIPITATION FROM MILK AND TREATED WITH A STRONG MINERAL ACID SELECTED FROM THE GROUP CONSISTING OF NITRIC ACID AND CHROMIC ACID WHILE UNIFORMLY DISPERSED IN AN AMMONIACAL AQUEOUS SOLUTION AND HAD A MINOR PART PRECIPITATED AND REMOVED FROM THE SAID SOLUTION. 